Differential long-term tamoxifen therapy benefit by menopausal status in breast cancer patients: secondary analysis of a controlled randomized clinical trial

Background Estrogen receptor–positive breast cancer patients have a long-term risk of distant metastatic disease, and premenopausal patients have a higher risk. Randomized studies with long-term follow-up are essential to understand treatment benefit. We elucidated the long-term tamoxifen therapy benefit by menopausal status in the Stockholm tamoxifen trials with 20 years complete follow-up. Methods Secondary analysis of 1242 estrogen receptor–positive and HER2-negative patients that were randomly assigned to 2-5 years of 40 mg adjuvant tamoxifen or no endocrine therapy. Distant recurrence-free interval in tamoxifen-treated vs endocrine untreated patients was assessed by Kaplan–Meier, Cox proportional hazards regression, and time-varying analyses. Results In premenopausal patients, a statistically significant tamoxifen benefit was observed for lymph node–negative (adjusted hazard ratio [HR] = 0.46, 95% confidence interval [CI] = 0.24 to 0.87), progesterone receptor–positive (adjusted HR = 0.61, 95% CI = 0.41 to 0.91), and genomic low-risk tumors (adjusted HR = 0.47, 95% CI = 0.26 to 0.85) but only lasted beyond 10 years for genomic low-risk tumors. Postmenopausal patients showed long-term benefit for all good-prognosis markers including low-grade (adjusted HR = 0.55, 95% CI = 0.41 to 0.73), lymph node–negative (adjusted HR = 0.44, 95% CI = 0.30 to 0.64), progesterone receptor–positive (adjusted HR = 0.60, 95% CI = 0.44 to 0.80), Ki-67 low (adjusted HR = 0.51, 95% CI = 0.38 to 0.68), and genomic low-risk tumors (adjusted HR = 0.53, 95% CI = 0.37 to 0.74), and regardless of tumor size (≤20 mm: adjusted HR = 0.55, 95% CI = 0.39 to 0.77; >20 mm: adjusted HR = 0.64, 95% CI = 0.44 to 0.94). Premenopausal patients with no poor-prognosis tumor characteristics (clinical marker score = 0) showed early benefit and postmenopausal long-term benefit. Conclusions Our study suggests differential tamoxifen benefit by menopausal status. Improved long-term endocrine therapy prediction in premenopausal patients is needed and could involve molecular markers because standard tumor characteristics cannot predict benefit beyond 10 years.